The present method is based on the so-called CVD (chemical vapor deposition) technique which consists of directing gaseous blasts of one or more reagents onto a substrate which has been brought to a high temperature, these gaseous blasts uniting on the substrate or in the immediate vicinity thereof and depositing thereon a solid product, in the form of a coating which is uniform to a greater or lesser extent and results from a decomposition or chemical reaction of the said reagents. In usual practice, the reactive gases are guided into the substrate by means of blast pipes arranged such that at the point where they meet the reactive gases are mixed in non-turbulent conditions, vortex motions likely to cause a premature reaction of the reactive gases and the deposition of part of the solid product on the ends of the blast pipes and the other parts of the coating device being avoided as far as possible. Parasitic depositions of this type foul the device and rapidly alter the operational characteristics which is extremely undesirable.
Thus, for example, the patent FR-A-2.348.165 (BFG GLASS GROUP) discloses a coating device (see page 10, lines 8-31; FIG. 2) comprising two blast pipes directed at 35.degree. maximum with respect to one another, in order that the gaseous flows emerging from these pipes mix progressively with a minimum of turbulence. In addition, for the same purpose, the gaseous blast resulting from the mixing of the above flows is directed onto the substrate to be coated at an angle which does not exceed 60.degree.. Other details on the previous CVD coating methods and devices may be found, for example, in the following references: FR 2 083 818; BE 887 465; U.S. Pat. No. 3,850,679; FR 2 314 152; CH 7 033/79.